Data writing and reading

ABSTRACT

Method and apparatus for recording data on a magnetic recording medium and writing data to a computer bus in which the data is divided into words of a predetermined length, a signal is generated for each data word, having an amplitude which is representative of the value of the data word, and this signal is then recorded to the magnetic recording medium or written to the computer bus. Complementary method and apparatus is provided for reading the data from the magnetic recording medium and the computer bus.

This application is the US national phase of international applicationPCT/GB01/04361 filed 28 Sep. 2001 which designated the U.S.

FIELD OF THE INVENTION

The invention relates to writing and reading data, for example within acomputer system.

BACKGROUND OF THE INVENTION

Data is required to be written and read in many sub-systems withincomputer systems. For example, in magnetic storage systems, data iswritten to a magnetic recording medium by a transducer which leaves amagnetic signature within the magnetic material of the recording medium.The data is then read from the magnetic material at a later stage by afurther transducer or in many cases the same transducer which detectsthe magnetic signature within the magnetic recording medium. Componentswithin a computer system also pass data in via bus systems. Such bussystems may interconnect internal adaptor cards, memory, video orperipheral host adaptor cards for example. Other bus systems mayinterconnect internal or external peripheral units such as scanners,printers or magnetic storage devices for example. A first componentwrites the data to the bus in the form of a series of changes in voltageor current. Components requiring the data read the data from the bus bydetecting these changes.

In all aspects of computer technology there is a continuous requirementfor more data storage, for data to be processed faster and for it to becommunicated faster. In presently known magnetic storage devices, databits are stored on the magnetic storage medium using a variety ofmethods. Such methods include the presence or absence of a magneticsignature on a given area of the medium, analogue frequencyrepresentation of corresponding data bits “1” and “0” or in longitudinalpatterns of magnetic particles with North-South or South-North magneticpolarity. Such an arrangement of representative data bit “1” and “0” inaccordance with the longitudinal magnetic orientation. In order toincrease the amount of data stored on a given area of magnetic medium,the data footprint size must be decreased, the number of recording sidesmust be increased and the recording tracks must be decreased in widthand located in closer proximity to each other. However as the arealdensity increases so does the likelihood of adjacent bits degrading eachother's magnetic signatures. In addition, if the bit area is to bereduced, the accuracy of the magnetic head and its positioning systemmust be correspondingly increased. A further problem for presentmagnetic storage devices is performance. Faster reading and writingoperations can increase performance. Performance increases can beachieved by increasing the speed of which the media surface (also knownas the substrate) is passed under the transducer used for writing andreading the magnetic data signatures. Currently, data words are writtenand read sequentially. Performance increases can be achieved byincreasing the rate at which the substrate is passed under thetransducer used for writing and reading the magnetic data signatures.However by writing and reading whole data words at a time would resultin a significant performance.

In buses of the type presently known bits are transmitted by beingwritten onto the bus at a first location by a first component in theform of the presence of an electric voltage or current and by being readat a second location by a second component which detects changes in theelectric voltage or the direction of the electric current. The timeinterval required to be left between bits is dependent on the signaldegradation which in turn is dependent on the cable length, signalshaping and electrical characteristics. As the bus length is determinedby several factors, such as signal shaping and electricalcharacteristics, the achievable bit rate on the bus is limited.Therefore in order to increase the data throughput on a bus it isnecessary to increase the bus size, i.e. provide further wires orenhance signal processing, etc. This requires more equipment whichincreases the cost and the complexity of the bus.

It is an aim of the present invention to alleviate some of the problemsdescribed above with conventional data reading and writing methods.

SUMMARY OF THE INVENTION

According to a first aspect of the first embodiment of the inventionthere is provided a method of reading data from a magnetic recordingmedium, said method comprising:

reading signals recorded on a magnetic medium;

generating data words each having a value which is represented by theamplitude of a read signal;

combining a plurality of data words to form a sequence of data.

According to a second aspect of the first embodiment of the inventionthere is provided a method of recording data onto a magnetic recordingmedium, said method comprising:

arranging data into words of a predetermined length;

generating for each said data word, a signal having an amplitude whichis representative of the value of the data word; and

recording said signals on the magnetic recording medium.

This is advantageous as it increases the bit density of the magneticstorage medium (and hence the capacity of a given magnetic storagemedium) without decreasing the size of the bit storage areas, orincreasing the areal density of the magnetic medium. This means that therisk of adjacent bit areas degrading each other's magnetic signatures isnot compromised and the accuracy of the magnetic head positioning doesnot need to be increased. The invention further allows the read-writespeed of the storage device to be increased without having to increasethe speed at which the magnetic medium is passed under the magnetichead. Similarly, the speed of the substrate can be reduced to preventlarge generations of heat or buffer overload. The speed of the substratemay need to be reduced from current levels in order to prevent thegeneration of heat from affecting the integrity of the data word.

In a preferred embodiment of the invention calibration signals arerecorded onto the predetermined regions of the magnetic recordingmedium. These calibration signals are subsequently read and used to seta plurality of expected signal amplitudes or a plurality of signalamplitude ranges which are used to determine which data word a readsignal amplitude represents.

The use of calibration signals is advantageous as it improves theaccuracy of the data read from the magnetic recording medium.

In a preferred embodiment of the invention a known signal is read from apredetermined portion of the magnetic recording medium and the noiseinherent in the signal is extracted. The inherent noise signal is theninjected into the data channel, thereby cancelling at least a portion ofthe noise in the read data signal.

This is advantageous since it improves the reading apparatus' ability todiscriminate between signals of different amplitudes which in turnincreases the number of amplitude levels which can be used.

According to further aspects of the first embodiment of the presentinvention there is also provided apparatus corresponding to the methodsdescribed above.

According to a first aspect of the second embodiment of the inventionthere is provided a method of reading data from a line of a computerbus, said method comprising:

reading signals written on a line of a computer bus;

generating data words each having a value which is represented by theamplitude of a read signal;

combining a plurality of data words to form a sequence of data.

According to a second aspect of the second embodiment of the inventionthere is a provided a method of writing data to a line of a computerbus, said method comprising:

arranging data into words of a predetermined length;

generating for each said data word, a signal having an amplitude whichis representative of the value of the data word; and

writing said signals to the line of the computer bus.

This embodiment of the invention is advantageous as it increases thedata rate attainable on each line of a computer bus. Therefore acomputer bus of the present invention may replace a conventionalcomputer bus using fewer lines but maintaining the same total data rateor the computer bus may be installed with various numbers of lines up toor more than the original number of lines providing a larger data rate.

In a preferred embodiment of the invention calibration signals arewritten onto the bus either at predetermined times or in response to acontrol signal. The calibration signals are subsequently read from thecomputer bus (either at the predetermined time or in response to acontrol signal) and are used to set a plurality of expected signalamplitudes or a plurality of signal amplitude ranges which are used todetermine which data word a read signal amplitude represents

The use of calibration signals is advantageous as it improves theaccuracy of the data read from the computer bus. If the calibrationsignals are written onto the computer bus in response to a controlsignal then it is possible to re-calibrate if an increase in the biterror rate is detected.

According to further aspects of the second embodiment of the presentinvention there is also provided apparatus for performing the methodsdescribed above.

The invention will now be described by way of non-limiting examples withreference to the accompanying drawings.

IN THE DRAWINGS

FIG. 1 shows a table of example 8 bit data words and the correspondingamplitudes of signals used in an example of the invention;

FIG. 2 shows the example 8 bit data words of FIG. 1, graphicallyrepresented;

FIG. 3 a shows a schematic representation of apparatus according to thefirst embodiment of the invention for writing data to a magneticrecording medium;

FIG. 3 b shows a schematic representation of apparatus according to thefirst embodiment of the invention for reading data from a magneticrecording medium;

FIG. 4 a shows a magnetic recording medium of a first type for use withthe first embodiment of the invention;

FIG. 4 b shows a magnetic recording medium of a second type for use withthe first embodiment of the invention;

FIG. 5 a shows a schematic representation of apparatus according to thesecond embodiment of the invention for writing data to a computer bus;

FIG. 5 b shows a schematic representation of apparatus according to thesecond embodiment of the invention for reading data from a computer bus.

FIG. 6 shows a schematic representation of apparatus according to avariation of the first embodiment of the invention for reading data froma magnetic recording medium.

DETAILED DESCRIPTION

In the present invention a stream of input data is divided into datawords which may be of the same length as or a different length from theword length of the input data. Each data word, which is distinguished byits value, is represented by a different quantized analogue signal andeach quantized analogue signal only represents that data word. If thedata word contains 2 bits there are four possible states: 00, 01, 10,11. These four data words are represented by signals with four differentamplitudes, for example which are proportional to the value of the datawords. In the example given the signals' amplitudes would be onequarter, two quarters, three quarters and the maximum possible signalamplitude. However there may be circumstances in which it is desirableto assign the data words to the quantized analogue signals in otherways, for example randomly or quasi-randomly. In addition some of thesignal amplitudes may be assigned to provide extra information, such assector number, track number,-,address mark or error check information.

The writing device determines which of the four possible data words isnext in the data stream and writes a signal at the correspondingamplitude. The reading apparatus, when it reads a signal, determines therelative amplitude of the signal and outputs the corresponding two-bitdata word. Thus the system is able to write or read the signal two bitsat a time compared to the conventional one bit at a time which improves,by a factor of two, the rate at which data can be written and read. Inthe case of writing and reading data to and from a magnetic recordingmedium this also increases, by the same factor, the data storagecapacity of the medium. Larger data words can be used to secure largerperformance improvements.

The table below (Table 1) shows examples of the capacity improvementsthat may be achieved using the present invention when applied tomagnetic recording media. The example given is a Fujitsu M8100—128-TrackTape Drive used with a sustained transfer rate of 13.5 Mbps over a SmallComputer Systems Interface (SCSI) bus.

TABLE 1 Current 4 bit data 8 bit data 12 bit data 16 bit data Form wordswords words words Capacity 10 GB 40 GB 80 GB 120 GB 160 GB Bit Density“Bits per 86,360 345,440 690,880 1,036,320 1,381,760 inch” (BPI)Transfer Rate Sustained 13.5 13.5 13.5 13.5 13.5 (Mbps) over SCSIInterface Media Type 3590 ¼″ 3590 ¼″ 3590 ¼″ 3590 ¼″ 3590 ¼″ Tape TapeTape Tape Tape

The bit density is measured in Bits per Inch. The improvements in thecapacity of the tape store and resulting bit density increase inproportion to the size of the data words used. It is clear that thepresent invention results in marked improvements.

Table 2, below, shows the improvements in the transfer rate sustainedwhich can be achieved with the present invention, again using a FujitsuM8100—128-Track Tape Drive, but this time replacing the SCSI HardwareInterface. To illustrate the impact of these improvements, the tablealso shows the time that would be required to deliver 10 GBs of data tothe backup drive unit (assuming that the host computer can deliver therequired sustained throughput). Corresponding improvements in thetransfer rate of a data bus can be achieved when the present inventionis applied to a computer bus.

TABLE 2 SCSI 4 bit data 8 bit data 12 bit data 16 bit data Interfacewords words words words Transfer Rate Sustained 13.5 54 108 162 216(Mbps) Time to complete 10 GBs in 12:00 03:15 01:30 01:00 00:47 MM:SS(approx)

FIG. 1 shows a table of example values for the present invention inwhich 8 bit data words are written or read simultaneously. In this casethe amplitude of the signal is quantized to one of 256 equally spacedlevels (0 to 255). FIG. 2 shows the examples in graphical form. It canbe appreciated that as the size of the data words written or read in asingle step increases the required accuracy of the writing and readingapparatus increases exponentially. Notwithstanding this, the inventionis applicable to data words of any size. However, data word sizes arelimited by factors other than those directly imposed by the presentinvention. Such factors, limiting data word size include:

the precision and quality of electronic components used to construct theinvention;

the tolerance of a magnetic medium to facilitate recording of a spectrumof amplitude steps;

heat factors and their effect on the integrity of the data.

FIGS. 3 a and 3 b represent the writing and reading apparatus,respectively, of a first embodiment of the invention. In this examplebinary data is written to and read from a hard drive of a computer. Itshould be noted however that this, embodiment of the present inventionis applicable to any circumstance in which data is recorded onto amagnetic recording medium.

At the input interface 1 between the existing computer hardware and thewriting apparatus of the present invention the data may be either serialor parallel and, in the case that the data input is parallel, may be anyof two or more bits. The number of bits in the data word used in thewriting and reading may be predetermined or may be set by a controlsignal (not shown) and is not limited to being the same size as the datawords of a parallel data input. The input interface 1 therefore containsa data buffer to enable it to arrange the data into data words of thecorrect size. In the case that the data is in parallel format with thesame number of bits as is contained in the data words then the divisionperformed in the buffer will be implicit.

The data words are passed from the input interface 1 to the binary tovoltage converter 3 which determines which data word has been receivedand outputs the corresponding quantized analogue signal, a voltagesignal at a proportion of the maximum voltage. The binary to voltageconverter 3 operates in a similar manner to conventional Digital toAnalogue converters such as Digital to Analogue converter IntegratedCircuits, R-2R ladder converters or binary weighted circuits. The outputfrom the binary to voltage converter is fed to the amplification andgain control circuit 4 which amplitude modulates a carrier signalgenerated by the frequency generator 2. The carrier signal is set at afrequency which can be successfully recorded on the magnetic recordingmedium 6. The output of the amplification and gain control circuit 4 isthe carrier signal at an amplitude determined by the data wordoriginally inputted. The writing transducer 5 then records the signal tothe magnetic recording medium.

FIG. 3 b shows the reading apparatus which is complementary to thewriting apparatus described above. The magnetic signal written on themagnetic recording medium 6 is read by the reading transducer 13 andtransferred to the calibration and amplification circuit 12 in which thesignal is demodulated to produce an amplitude signal. The amplificationto binary converter circuit 11 compares the amplitude signal to theexpected signal levels, which may be set by calibration tracks asdiscussed below, to determine which of the data words the signalrepresents. The corresponding data word is then outputted from theamplification to binary circuit 11 to the output interface 10 betweenthe reading apparatus of the present invention and the existing computerhardware. As with the input interface 1, the output interface 10 mayoutput the binary data in either parallel or sequential form. A bufferin the output interface enables this as well as permitting the use ofdata words of a different size to the size to the output.

In a variation of this embodiment of the invention, the quantizedanalogue signal may be recorded directly onto the magnetic recordingmedium without modulating a carrier signal.

In a further variation of this embodiment an opto-magneto apparatus isused. In such an apparatus a laser is used to heat the substrate of amagnetic recording medium immediately prior to the transducer recordingthe signal. The heating reduces the amount of magnetic flux required bythe transducer to polarise “in a North-South or South-North fashion” isreduced. This is safeguards the integrity of adjacent data bits as thereis less magnetic flux infringing upon neighbouring data bits where highareal density has been achieved. The magnetic particles in the heatedregion are polarised longitudinally to an angle which is dependent onthe “write” current used. According to the present invention theopto-magneto apparatus can be used by using the writing and readingapparatus described above without the use of the carrier signal (or, putanother way the frequency of the carrier signal is set to zero). Thus acomplete data word is represented by the magnitude of the “write signal”and, on the medium, by the angle of the polarised magnetic particles.

FIG. 4 a shows a section of a magnetic tape 20 which forms an example ofa suitable medium for recording signals as described above. The tape 20is split into discrete recording tracks 21. In a preferred embodimentsome of the tracks will be data-containing tracks 22 and the remainingtracks or sections of tracks 23 will be for calibration, sector number,track number, address mark and error check information, etc. FIG. 4 bshows a corresponding disc-shaped magnetic recording medium 30 which hasrecording tracks 31 which may be used as data-containing tracks 32 or ascalibration tracks 33. The magnetic discs or tapes may be used in harddrives or be contained in removable cassettes, such as “floppy disks”,“Zip disks”, “Jaz disks” and “DAT tapes”, for use in computers or otherelectronic equipment.

The calibration tracks 23, 33 may be used to improve the accuracy of thewriting and subsequent reading process. Calibration signals representingpredetermined data words are recorded at known sections of thecalibration tracks 23, 33. The calibration signals are subsequently readby the reading circuit 101 and stored for use in the amplification tobinary converter 11. The reading circuit 101 is instructed by a controlunit (not shown) to read the calibration signals. This may occur atpredetermined intervals, may be triggered to occur as a result of anincrease in the bit error rate of the read data or may occur as a resultof either.

The calibration signals may be used to provide a plurality of signalswith amplitudes representing the possible data words. Data signals arethen compared to the calibration signals to determine which data wordthe data signals read from the data-containing tracks 22, 32 represent.Alternatively the calibration signals may be used to set signalamplitude ranges. A read signal which is within a set amplitude range isthen be recognised as representing a particular data word. A read signalwhich does not fall within any amplitude range could then be recognisedas incorrect or uncertain and be subsequently re-read or re-processed orboth. Preferably the calibration tacks 23, 33 are recorded onto themedium when the writing apparatus records the data-containing tracks orwhen the medium is first formatted. Known format operations includewriting track information, sector information, address mark information.

The calibration is especially useful since the method and apparatus ofthe present invention rely on non-saturation magnetic recording and eachmagnetic medium will respond slightly differently to a given input. Thisvariation, which may be caused by variations between magnetic mediums,by variations between both recording apparatus and reading apparatus andmay also change with time, may be large enough that, withoutcalibration, signals may be incorrectly read as representing aparticular data word.

An additional improvement can be made in the reading process bycompensating for the noise in the signal read from the magneticrecording medium. Inherent noise may be present in the signal due to,for example, variations in the grain size and grain density in themagnetic medium. This is especially true for magnetic tapes in which thestretching of the tape caused by the tension of the tape causesadditional variation.

FIG. 6 shows a variation of the reading apparatus 105 of the firstembodiment of the present invention. The apparatus includes an analoguebuffer 7. A known noise compensation signal is read from a predeterminedportion of the magnetic recording medium 6 and from this the inherentnoise is extracted. The known signal may be read from a portion of themagnetic recording medium on which no signal has been recorded. Toextract the inherent noise in this case, no further processing will berequired. Alternatively it may be a signal recorded at a known level,such as one of the calibration signals. In the latter case, thedifference, between the read signal and the signal known to have beenrecorded will be the inherent noise. Therefore to extract the inherentnoise in this case it will be necessary to deduct the known signal levelfrom the noise compensation signal. A sample of the inherent noiseextracted from the noise compensation signal is then stored in theanalogue buffer. The predetermined portion of the magnetic recordingmedium may be a designated track, part of a track, a sector or part of asector depending on the type of magnetic recording medium.

During the read operation, the sample held in the analogue buffer 7 canbe injected into the data channel. In doing so it will cancel at least aportion of the inherent noise in the data channel. The analogue buffer 7is provided with a switch 8 which can be used to apply the sample in theanalogue buffer as required.

The analogue buffer may be comprised of components such as aBucket-Brigade Device (BBD), Delay Line, Analogue Delay Line, PassiveAnalogue Delay Line, Ditally Programmable Analogue Delay Line orTrimable Analogue Delay Line. It may also be comprised of componentsthat have a hybrid construction utilising proven technologies of activeintegrated circuitry and of passive networks utilizing capacitive,inductive and resistive elements. The sample in the analogue buffer canbe continually re-inputted to itself by means of a loop system. Thesample in the buffer can be updated by reading a new known signal from apredetermined portion of the magnetic recording medium periodically orin response to a control signal triggered, for example, by an increasein the bit error rate.

In an alternative apparatus the known signal is continuously read from apredetermined portion of the magnetic recording medium thereby obviatingthe need for the analogue buffer.

The improvements of this invention to devices for recording data onmagnetic storage mediums may be applied to designing entirely newmagnetic data storage devices. However it is also intended that existingdevices can be modified to incorporate the present invention. Forinstance this may be possible by installing a new card which interfaceswith the internal data bus and utilises the existing magnetic medium,transducers and other mechanics.

A second embodiment of the invention is used to read and write data toand from a computer bus system. The invention may be used with eitherinternal or external computer buses. Internal computer buses are used tohost Adaptor and Add-on system enhancement cards which are used, forexample, to provide additional memory or improved graphics processing.External, or peripheral, buses are used to connect peripheral devicessuch as image scanners, printers and magnetic tape backup devices.Physically the computer buses are, comprised of a set of wires (lines)to which all of the components or devices on the bus are attached.Typically some of the wires are used as control lines for controllingaccess to the bus and the remainder are used as data lines. The busesused -in the present invention may require some RF shielding in order toimprove the signal accuracy.

The data writing circuit 102 and the data reading circuit 103, shown inFIGS. 5 a and 5 b respectively, are identical to those used for writingand reading data to a magnetic storage medium described above except forthe replacement of transducers 5, 13 by connections 45, 53 to the databus. Therefore the description thereof is not repeated. Thus, in thedata writing circuit 102 the modulated carrier signal which is outputtedfrom the amplification and gain control circuit 4, and represents theinputted data word, is written onto one of the data lines of the bus 46.The signal from the bus 46 is read into the data reading circuit 103 viathe calibration and amplification circuit 12. In this manner each lineof the bus is capable of transmitting a complete data word rather thaneither having to transmit the data word one bit at a time or use aseveral lines to transmit the entire data word in one go., Although notshown in FIGS. 5 a and 5 b, the data writing circuit 102 and the datareading circuit 103 will also have connections to the bus' controllines.

In a variation of this embodiment the quantized analogue signal may bewritten directly to the computer bus without modulating a carriersignal. In a further variation of this embodiment the quantized analoguesignal may be the amplitude of an electric current rather than theamplitude of a voltage.

Calibration of the circuits 102, 103 may take place either atpredetermined time intervals or may be prompted by the bus exceeding apredetermined bit error rate. In the former case the data writingcircuit 102 writes predetermined calibration signals to the computer busat set times. In the latter case the data writing circuit 102 writes acalibration signal to the computer bus in response to a control signal(not shown) from the computer bus. The choice of calibration signalwritten to the computer bus may also be controlled by the control signalfrom the computer bus. The data reading circuit 103 reads thecalibration signals from the computer bus at the predetermined time, inresponse to the control signal or a combination of the two. Thecalibration signals are used in the same way as described for the firstembodiment in the amplification to binary converter circuit 11.

As with the embodiment for recording data onto magnetic recordingmediums, it is intended that this embodiment of the invention isapplicable to the upgrading of existing computer buses as well as to thedesign of new computer bus systems.

1. A method of reading data from a magnetic recording medium, saidmethod comprising: reading signals recorded on a magnetic medium;generating data words each having a value which is represented by theamplitude of a read signal; combining a plurality of data words to forma sequence of data; wherein said step of generating data wordscomprises: comparing the amplitude of the read signals to a plurality ofexpected signal amplitudes or a plurality of signal amplitude ranges inorder to determine which data word is represented by the read signal. 2.A method of reading data from a magnetic recording medium according toclaim 1, further comprising: reading a calibration signal having atleast one predetermined amplitude from a predetermined location on saidmagnetic recording medium; using the amplitude of the read calibrationsignal to set at least one of said expected signal amplitudes or saidsignal amplitude ranges.
 3. A method of reading data from a magneticrecording medium according to claim 1, further comprising: reading aknown noise compensation signal from a predetermined location on themagnetic recording medium; extracting the inherent noise in the noisecompensation signal; and adding the inherent noise extracted from thenoise compensation signal to the signals read from the magneticrecording medium.
 4. A method of reading data from a magnetic recordingmedium according to claim 3, wherein the extracted inherent noise isstored in an analogue buffer.
 5. A method of reading data from amagnetic recording medium according to claim 3, wherein the known noisecompensation signal is a calibration signal.
 6. A method of reading datafrom a magnetic recording medium according to claim 3, wherein the knownnoise compensation signal is read from a portion of the magneticrecording medium on which no signal has been recorded.
 7. A method ofrecording data onto a magnetic recording medium, said method comprising:arranging data into words of a predetermined length; generating for eachsaid data word, a signal having an amplitude which is representative ofthe value of the data word; and recording said signals on the magneticrecording medium; said method further comprising generating acalibration signal having at least one predetermined amplitude; andrecording said calibration signal at a predetermined location on saidmagnetic recording medium.
 8. A method of recording data onto a magneticrecording medium according to claim 7, wherein no signal is recorded ona predetermined portion of the magnetic recording medium.
 9. Anapparatus for reading data from a magnetic recording medium comprising:means for reading signals recorded on a magnetic recording medium; meansfor generating data words each having a value which is represented bythe amplitude of a read signal; and means for combining a plurality ofdata words to form a sequence of data; wherein said means for means forgenerating data words comprises: means for comparing the amplitude ofthe read signals to a plurality of expected signal amplitudes or aplurality of signal amplitude ranges in order to determine which dataword is represented by the read signal.
 10. An apparatus for readingdata from a magnetic recording medium according to claim 9, furthercomprising: means for causing a calibration signal, having at least onepredetermined amplitude, to be read from a predetermined location onsaid magnetic recording medium; and means for setting at least one ofsaid expected signal amplitudes or said signal amplitude rangesaccording to the amplitude of the read calibration signal.
 11. Anapparatus for reading data from a magnetic recording medium according toclaim 9, further comprising: means for causing a known noisecompensation signal to be read from a predetermined location on saidmagnetic recording medium; means for extracting the inherent noise inthe noise compensation signal; and means for adding the inherent noiseextracted from the noise compensation signal to the signals read fromthe magnetic recording medium.
 12. An apparatus for reading data from amagnetic recording medium according to claim 11, further comprising ananalogue buffer in which the extracted inherent noise is stored.
 13. Anapparatus for reading data from a magnetic recording medium according toclaim 11, wherein the known noise compensation signal is a calibrationsignal.
 14. An apparatus for reading data from a magnetic recordingmedium according to claim 11, wherein the known noise compensationsignal is read from a portion of the magnetic recording medium on whichno signal has been recorded.
 15. An apparatus for recording data onto amagnetic recording medium, comprising: means for arranging data intowords of a predetermined length; means for generating, for each saiddata word, a signal having an amplitude which is representative of thevalue of the data word; and means for recording said signals on themagnetic recording medium; further comprising: means for causing acalibration signal, having at least one predetermined amplitude, to begenerated and recorded at a predetermined location on said magneticrecording medium.
 16. An apparatus for recording data onto a magneticrecording medium according to claim 15, further comprising: means forcausing no signal to be recorded on a predetermined portion of themagnetic recording medium.